A methane fermentation treatment method in which an organic wastewater or an organic waste is decomposed by methane fermentation is superior with respect to energy savings as compared with an aerobic treatment such as an active sludge process, because it does not require an energy for aeration, it produces little surplus sludge and it permits energy recovery from a biogas generated. However, since the amount of proliferation of methane producing bacteria or methane fermentation bacteria is small and since sedimentation efficiency thereof is not high, the bacteria are apt to be discharged together with the treated water. Therefore, it is difficult to increase the concentration of bacteria within a fermentation reactor used for the methane fermentation treatment. Further, the methane fermentation treatment method has a problem with respect to costs and sites.
When a sulfur-containing waste water from a craft pulp cooking step, which contains sulfur-based foul odor substances, such as hydrogen sulfide and methyl mercaptan, that will hinder the methane fermentation, is treated by a medium temperature methane fermentation process using an anaerobic sludge blanket method, it is necessary to remove such foul odor substances and organic matters by steam stripping or air stripping. Thus, the above method has a problem because of increased operation costs.
Also known is a method which includes adding an agent for inhibiting the formation of hydrogen sulfide to a sulfate-containing organic wastewater, in which hydrogen sulfide has been formed, to produce a sulfide. The thus formed sulfide is then precipitated and removed from the wastewater. In the above method, the hydrogen sulfide is precipitated and removed as a sulfide formed by a heavy metal contained in the agent for inhibiting the formation of hydrogen sulfide. This method, however, has a problem, for example, that the methane fermentation is hindered by the heavy metal which is contained in the agent for inhibiting the formation of hydrogen sulfide which has been added in an excessive amount to the wastewater.
Thus, the conventional anaerobic active sludge treatment method for wastewater, such as chemical industry waster water (e.g. paper and pulp industry effluent), containing sulfur compounds, for example, hydrogen sulfide and methyl mercaptan, in high concentrations has the following problems.    (1) When a sulfur-containing organic substance is anaerobically decomposed, hydrogen sulfide is generated. When the pH is lowered, non-dissociable hydrogen sulfide, namely molecular hydrogen sulfide is generated. Such molecular hydrogen sulfide hinders methane fermentation.    (2) The excessive addition of the agent for removing hydrogen sulfide not only increases the process costs but also greatly damages anaerobic bacteria, when streaming in the methane fermentation reactor.
It is an object of the present invention to provide a high performance anaerobic methane fermentation treatment method and apparatus which can solve the problems of the above-described conventional technique and which can be applied to a sulfur compound-containing wastewater.